The present invention relates in general to the field of proton exchange membrane (xe2x80x9cPEMxe2x80x9d) fuel cell systems, and more particularly, to an improved PEM fuel cell system having improved discrete fuel cell modules.
A fuel cell is an electrochemical device that converts fuel and oxidant into electricity and a reaction by-product through an electrolytic reaction that strips hydrogen molecules of their electrons and protons. Ultimately, the stripped electrons are collected into some form of usable electric current by resistance or by some other suitable means. The protons react with oxygen to form water as a reaction by-product.
Natural gas is the primary fuel used as the source of hydrogen for a fuel cell. When natural gas is used, it must be reformed prior to entering the fuel cell. Pure hydrogen may also be used if stored correctly. The products of the electrochemical exchange in the fuel cell are DC electricity, liquid water, and heat. The overall PEM fuel cell reaction produces electrical energy equal to the sum of the separate half-cell reactions occurring in the fuel cell, less its internal and parasitic losses. Parasitic losses are those losses of energy that are attributable to any energy required to facilitate the ternary reactions in the fuel cell. Examples include power to drive pumps and run reformation equipment.
Although fuel cells have been used in a few applications, engineering solutions to successfully adapt fuel cell technology for use in electric utility systems have been elusive. The challenge is to generate power in the range of 1 to 100 kW that is affordable, reliable, and requires little maintenance. Fuel cells would be desirable in this application because they convert fuel directly to electricity at much higher efficiencies than internal combustion engines, thereby extracting more power from the same amount of fuel. This need has not been satisfied, however, because of the prohibitive expense associated with such fuel cell systems. For example, the initial selling price of the 200 kW PEM fuel cell was about $3500/kW to about $4500/kW. For a fuel cell to be useful in utility applications, the life of the fuel cell stack must be a minimum of five years and operations must be reliable and substantially maintenance-free. Heretofore known fuel cell assemblies have not shown sufficient reliability and have disadvantageous maintenance issues. Despite the expense, reliability, and maintenance problems associated with known fuel cell systems, there remains a clear and present need for economical and efficient fuel cell technology for use in residential and light-commercial applications because of their environmental friendliness and operating efficiency.
Fuel cells are usually classified according to the type of electrolyte used in the cell. There are four primary classes of fuel cells: (1) proton exchange membrane (xe2x80x9cPEMxe2x80x9d) fuel cells, (2) phosphoric acid fuel cells, and (3) molten carbonate fuel cells. Another more recently developed type of fuel cell is a solid oxide fuel cell. PEM fuel cells, such as those in the present invention, are low temperature low pressure systems, and are, therefore, well-suited for residential and light-commercial applications. PEM fuel cells are also advantageous in these applications because there is no corrosive liquid in the fuel cell and, consequently, there are minimal corrosion problems.
Characteristically, a single PEM fuel cell consists of three major componentsxe2x80x94an anode gas dispersion field (xe2x80x9canodexe2x80x9d); a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d); and a cathode gas and liquid dispersion field (xe2x80x9ccathodexe2x80x9d). As shown in FIG. 1, the anode typically comprises an anode gas dispersion layer 502 and an anode gas flow field 504; the cathode typically comprises a cathode gas and liquid dispersion layer 506 and a cathode gas and liquid flow field 508. In a single cell, the anode and the cathode are electrically coupled to provide a path for conducting electrons between the electrodes through an external load. MEA 500 facilitates the flow of electrons and protons produced in the anode, and substantially isolates the fuel stream on the anode side of the membrane from the oxidant stream on the cathode side of the membrane. The ultimate purpose of these base components, namely the anode, the cathode, and MEA 500, is to maintain proper ternary phase distribution in the fuel cell. Ternary phase distribution as used herein refers to the three simultaneous reactants in the fuel cell, namely hydrogen gas, water vapor and air. Heretofore known PEM fuel cells, however, have not been able to efficiently maintain proper ternary phase distribution. Catalytic active layers 501 and 503 are located between the anode, the cathode and the electrolyte. The catalytic active layers 501 and 503 induce the desired electrochemical reactions in the fuel cell. Specifically, the catalytic active layer 501, the anode catalytic active layer, rejects the electrons produced in the anode in the form of electric current. The oxidant from the air that moves through the cathode is reduced at the catalytic active layer 503, referred to as the cathode catalytic active layer, so that it can oxidate the protons flowing from anode catalytic active layer 501 to form water as the reaction by-product. The protons produced by the anode are transported by the anode catalytic active layer 501 to the cathode through the electrolyte polymeric membrane.
The anode gas flow field and cathode gas and liquid flow field are typically comprised of pressed, polished carbon sheets machined with serpentine grooves or channels to provide a means of access for the fuel and oxidant streams to the anode and cathode catalytic active layers. The costs of manufacturing these plates and the associated materials costs are very expensive and have placed constraints on the use of fuel cells in residential and light-commercial applications. Further, the use of these planar serpentine arrangements to facilitate the flow of the fuel and oxidant through the anode and cathode has presented additional operational drawbacks in that they unduly limit mass transport through the electrodes, and therefore, limit the maximum power achievable by the fuel cell.
One of the most problematic drawbacks of the planar serpentine arrangement in the anode and cathode relates to efficiency. In conventional electrodes, the reactants move through the serpentine pattern of the electrodes and are activated at the respective catalytic layers located at the interface of the electrode and the electrolyte. The actual chemical reaction that occurs at the anode catalyst layer is: H2⇄2H++2exe2x88x92. The chemical reaction at the cathode catalyst layer is: 2H++2exe2x88x92+xc2xdO2⇄H2O. The overall reaction is: H2+xc2xdO2⇄H2O. The anode disburses the anode gas onto the surface of the active catalyst layer comprised of a platinum catalyst electrolyte, and the cathode disburses the cathode gas onto the surface of the catalytic active layer of the electrolyte. However, the anode gas and the cathode gas are not uniformly disbursed onto the electrolyte when using a conventional serpentine construction. Non-uniform distribution of the anode and cathode gas at the membrane surface results in an imbalance in the water content of the electrolyte. This results in a significant decrease in efficiency in the fuel cell.
The second most problematic drawback associated with serpentine arrangements in the electrodes relates to the ternary reactions that take place in the fuel cell itself. Serpentine arrangements provide no pressure differential within the electrodes. This prohibits the necessary ternary reactions from taking place simultaneously. This is particularly problematic in the cathode as both a liquid and a gas are transported simultaneously through the electrode""s serpentine pattern.
Another shortcoming of the conventional serpentine arrangement in the anode in particular is that the hydrogen molecules resist the inevitable flow changes in the serpentine channels, causing a build-up of molecular density in the turns in the serpentine pattern, resulting in temperature increases at the reversal points. These hot spots in the serpentine arrangement unduly and prematurely degrade the catalytic active layer and supporting membrane.
In the typical PEM fuel cell assembly, a PEM fuel cell is housed within a frame that supplies the necessary fuel and oxidant to the flow fields of the fuel cell. These conventional frames typically comprise manifolds and channels that facilitate the flow of the reactants. However, usually the channels are not an integral part of the manifolds, which results in a pressure differential along the successive channels. FIG. 2 is an illustration of a conventional frame for the communication of the reactants to a fuel cell. This pressure differential causes the reactants, especially the fuel, to be fed into the flow fields unevenly, which results in distortions in the flow fields causing hot spots. This also results in nonuniform disbursement of the reactants onto the catalytic active layers. Ultimately, this conventional method of supplying the necessary fuel and oxidant to a fuel cell results in a very inefficient process.
As a single PEM fuel cell only produces about 0.30 to 0.90 volts D.C. under a load, the key to developing useful PEM fuel cell technology is being able to scale-up current density in individual PEM cell assemblies to produce sufficient current for larger applications without sacrificing fuel cell efficiency. Commonly, fuel cell assemblies are electrically connected in nodes that are then electrically connected in series to form xe2x80x9cfuel cell stacksxe2x80x9d by stacking individual fuel cell nodes. Two or more nodes can be connected together, generally in series, but sometimes in parallel, to efficiently increase the overall power output.
These internal foils are often fabricated from graphite or metal. In conventional designs, the bonds between the internal foil and the anode gas flow field and the cathode gas and liquid flow field cause degradation of both the plate and the gas flow fields. This the results in efficiency losses in the fuel cell.
Conventional PEM fuel stacks often flood the cathode due to excess water in the cathode gas flow field. Flooding occurs when water is not removed efficiently from the system. Flooding is particularly problematic because it impairs the ability of the reactants to adequately diffuse to the catalytic active layers. This significantly increases the internal resistance of the cathode which ultimately limits the cell voltage potential. Another problem is dehydration of the polymeric membranes when the water supply is inadequate. Insufficient supply of water can dry out the anode side of the PEM membrane electrolyte, causing a significant rise in stack resistance and reduced membrane durability.
Further, conventional PEM fuel cells and stacks of such fuel cell assemblies are compressed under a large load in order to ensure good electrical conductivity between cell components and to maintain the integrity of compression seals that keep various fluid streams separate. In conventional fuel cells, all of the fuel cells for the fuel cell system are arranged in one stack. These conventional fuel cell assemblies are arranged horizontally between stack end plates. The assemblies in the center of the stack have a tendency to sag due to gravity, pulling them out of alignment and reducing efficiency or forcing a maintenance shutdown. To prevent this sag, designers increase the recommended compressive pressure on the compressive assembly. These compressive forces are often extreme, generally in excess of 40,000 psi, and use compression assemblies, such as tie rods and end plates. If tie rods are used, the tie rods generally extend through holes formed in the peripheral edge portion of the stack end plates and have associated nuts or other fastening means assembling the tie rods to the stack assembly to urge the end plates of the fuel stack assembly toward each other. Typically, the tie rods are external, i.e., they do not extend through the fuel cell electrochemically active components. This amount of pressure that must be used to ensure good electrochemical interactions presents many operational difficulties. For example, if the voltage of a single fuel cell assembly in a stack declines significantly or fails, the entire stack must be taken out of service, disassembled, and repaired, resulting in significant repair costs and down-time. Second, inadequate compressive force can compromise the seals associated with the manifolds and flow fields in the central regions of the interior distribution plates, and also compromise the electrical contact required across the surfaces of the plates and MEAs to provide the serial electrical connection among the fuel cells that make up the stack. Third, the extreme compressive force used unduly abrades the surfaces of the fuel cell modules within the stack, resulting in wear of components in the fuel cell assemblies such as the catalyst layers of the electrolyte, thereby leading to increased losses in fuel cell stack and fuel cell assembly efficiency.
One of the maintenance problems associated with conventional fuel cells is replacement of failed fuel cell, modules. Because conventional fuel cell modules are often arranged into one tall stack, a failed fuel cell module necessitates unstacking the modules to reach the failed module, replacement of the failed module, and finally, restacking the modules. Typical existing fuel cell assemblies require that the entire stack be out of operation during this exercise that may require several hours. Electricity is not generated during this maintenance operation.
Another of the disadvantages associated with the organization of fuel cell assemblies into tall stacks are the costs and problems associated with internal support of the weight of the fuel cell assemblies. Typical fuel cell assemblies are arranged so that an individual fuel cell assembly must support the weight of all of fuel cell assemblies above it. The stack of fuel cell assemblies exert a large downward force on the bottom fuel cell modules of the stack. Fuel cell assemblies must be designed to accommodate this force, and so are made thicker and more resilient than would otherwise be necessary. This additional thickness of the individual fuel cell assemblies makes the assemblies more expensive. Further, a higher motive force is necessary to drive reactants and the reacted product through the fuel cell. A higher motive force requires larger pumps and larger pipe sizes to move the reactants and product, driving up the amount of power needed to run the fuel cell assembly. This is commonly referred to as a higher parasitic load. The higher the parasitic load, the less electricity is available to users and devices outside of the internal workings of the fuel cell assembly.
The serpentine patters of the typical fuel cell assemblies and stacks also increase parasitic loads. Pumps must drive fluids through the labyrinthine patterns of conventional fuel cells, patterns that often consist of capillary tubes. Often, fluids must pass through multiple cell modules in series. This tortured flow path makes larger pumps and lines necessary, further increasing costs and parasitic loads. Larger balance of plant assemblies and the arrangement of traditional fuel cell plants increase the size of the footprint of the overall fuel cell plant. This may be undesirable, typically in residential settings where homeowners view large fuel cell plants as unsightly.
An economical and efficient fuel cell assembly and fuel cell stack assembly are provided herein that have optimized supply systems and mass transport systems.
Most broadly, the present invention is a fuel cell system with a reformer subassembly and a plurality of fuel cell stacks. The fuel cell stacks are so arranged as to substantially surround the reformer assembly with each fuel cell stack having a plurality of fuel inlet channels. Each fuel cell stack also has an fuel inlet channel, an air inlet channel and an air and water outlet channel. An air supply header connects a means for supplying air to the fuel cell stack with the air inlet channel of each fuel cell stack. Further, an air and water return header serves to discharge air and water from the air and water outlet channel. A fuel supply header serves to connect the reformer subassembly to the top of the plurality of fuel inlet channels of each fuel cell stack. A fuel discharge header is connected to the bottom of the fuel inlet channel of each fuel cell stack.
In another embodiment of the present invention, the fuel cell stack of the fuel cell system contains two end plates aligned with each other, a plurality of fuel cell assemblies interposed between the first end plate and the second end plate. Each fuel cell assembly is composed of an open-cell foamed gas flow field, an open-cell foamed gas and liquid flow field, and an MEA. The fuel cell assembly further has an internal foil. The internal foil has an anode connection surface and a cathode connection surface with the anode connection surface bonded to the gas flow field of a first fuel cell assembly and the cathode connection surface bonded to the gas and liquid flow field of a second fuel cell assembly. The fuel cell stack further has a compression assembly.
One advantage of the present invention is that fuel cell stack replacement is simplified. Because individual fuel cell stacks may be removed and replaced, rather than unstacked and then restacked, maintenance operations for failed fuel cell assemblies are quicker, with less down time.
Another advantage of the present invention is the ease in altering the fuel cell system to accommodate a need to increase or decrease voltage as the individual fuel cells are externally strapped allowing both series and parallel connections. Because of the system architecture, voltage capacity may be added to each fuel cell stack merely by adding fuel cell assemblies to each stack.
Still another advantage of the present invention is that parasitic loads are reduced by reducing pressure drop. Because the weight of each fuel cell assembly has been reduced, each fuel cell assembly may be made thinner, reducing the need for a larger motive force to move hydrogen and air through the fuel cell assemblies. This results in smaller pumps and blowers for bulk transport of reactants and products. In addition, because the reformer is centrally located, distances to each fuel cell stack are minimized, further reducing pressure drop and the need for a large motive force. Also, because of the fuel supply and air supply channels created in the present invention, large pressure drops are eliminated in delivering the fuel and air to the fuel cell itself for air and fuel supply, as well as liquid removal. The central location of the reformer also facilitates heat management.
Yet another advantage of the present invention is the small footprint of the fuel cell system. Because of the central location of the reformer, and the reduced sizes pumps and blowers, the fuel cell system may be made more compact.
Another advantage of the present invention is that it may load follow. Because there is no serpentine arrangement in the gas and gas and liquid flow fields, an increase in demand for electricity may be quickly met by increasing flow of fuel and air.
Another advantage is that the invention achieves optimal mass transport through the open cell foamed electrodes and the distribution frame of the present invention. This increases the overall efficiency and maximum power achievable by the fuel cell. Further, the anode gas and cathode gas are uniformly disbursed on the electrolyte, resulting in optimal water balance in the fuel cell system. Flooding and drying out of the electrolyte are thereby avoided.
Other advantages of the present invention will be apparent to those ordinarily skilled in the art in view of the following specification claims and drawings.